Edit

When navigating the taxontree the small icons to close/expand the subtree disappear. The regex misses to match a ".

cdm_taxontree.js
195: var bg_image_tmp = jQuery(this).css('background-image');
196: var bg_image_new = bg_image_tmp.replace(/^.*(\/.*)())$/, '$1/' + imageFile + '$3');
197: if (jQuery.browser.mozilla) {
198: // Special bug handling for mozilla: strip of last closing bracket.
199: bg_image_new = bg_image_new.substr(0, bg_image_new.length - 1);

This is actually not a bug in mozilla.

bg_img_tmp: url("http://portal.cybertaxonomy.org/flora-guianas/sites/flora-guianas/modules/cdm_dataportal/cdm_taxontree/plus.png")
bg_image_new: url("http://portal.cybertaxonomy.org/flora-guianas/sites/flora-guianas/modules/cdm_dataportal/cdm_taxontree/loading_subtree.gif)

bg_image_new actually misses a " before the closing parenthesis, because it's matched in group $2. The "fix" removes enough from the string that firefox can autofix it.
Chrome throws something like this:
GET http://portal.cybertaxonomy.org/flora-guianas/sites/flora-guianas/modules/cdm_dataportal/cdm_taxontree/minus.png) 404 (Not Found)

Notice the closing bracket in the end.